High energy density secondary lithium batteries

ABSTRACT

A lithium ion battery includes a positive electrode comprising carbon fibers, a binder composition with conductive carbon, and a lithium rich composition. The lithium rich composition comprises at least one selected from the group consisting of Li 1+x (My Mz II  Mw III )O 2  where x+y+z+w=1, and xLi 2 MnO 3 (1−x)LiMO 2 , where x=0.2-0.7, and where M, M II  and M III  are interchangeably manganese, nickel and cobalt, and LiM 2−x M x   II O 4 , where M and M II  are manganese and nickel, respectively, with x=0.5. A negative electrode comprises carbon fibers having bound thereto silicon nanoparticles, and a mesophase pitch derived carbon binder between the silicon nanoparticles and the carbon fibers. An electrolyte is interposed between the positive electrode and the negative electrode. Methods of making positive and negative electrodes are also disclosed.

This invention was made with government support under contract No.DE-AC05-00OR22725 awarded by the U.S. Department of Energy. Thegovernment has certain rights in this invention.

FIELD OF THE INVENTION

This invention relates generally to lithium ion batteries, and moreparticularly to electrodes for lithium ion batteries and methods formaking electrodes for high energy density lithium ion batteries.

BACKGROUND OF THE INVENTION

A large variety of materials have been synthesized and evaluated ascathode materials for Li-batteries. Notable among them is the layeredLiMO₂ (M=Co, Ni, Mn) compositions which have already found applicationin rechargeable lithium ion battery technology. However, only about50-60% of the theoretical capacity can be utilized in practical cellsbecause of structural and chemical instabilities associated with deepcharge of Li_(1−x)MO₂(x>0.5) along with safety issues. In order toincrease the energy density, recent developments have focused on thelithium rich Li—Ni—Mn—Co oxide compounds that have significantly highercapacities. See for example Thackeray et al U.S. Pat. No. 7,135,252 andUS 2006/0099508, the disclosures of which are incorporated by reference.These materials can be represented using either (i) structurallyintegrated two-component solid solution notations such asxLi₂MnO₃(1−x)LiMO₂ (layered-layered in which the Li₂MnO₃ component iselectrochemically activated above 4.4 V vs. Li/Li⁺) or (ii) standardnotation as Li_(1+y)M_(1−y)O₂ (M=Mn, Ni, Co). For example, thecomposition0.6Li[Li_(1/3)Mn_(2/3)]O₂-0.4Li[Mn_(0.3)Ni_(0.45)Co_(0.25)]O₂ (hereafterLi-rich MNC) can be alternately expressed asLi_(1.2)Mn_(0.525)Ni_(0.175)Co_(0.1)O₂ in the standard notation for suchlayered compositions. Electrodes based on these Li-rich MNC compositionscan operate at high anodic potentials of 4.9 V vs. Li/Li⁺ and providecapacities >250 mAh g⁻¹. There are still major issues that need to beaddressed before these lithium rich compounds can be considered ashigh-energy cathodes for production Li-ion batteries, especially forelectric vehicle applications. Notable among these are poor ratecapability, high first cycle irreversibility and significant decrease inthe discharge voltage plateau with successive cycling. The largeirreversible capacity loss in the range of 50-100 mAh g⁻¹ in the firstcycle is attributed to the extraction of Li₂O followed by elimination ofoxygen ion vacancies from the lattice during first charge, resulting ina lower number of sites for insertion and extraction of Li⁺ in thesubsequent cycles. Further, detailed structural and phase transitionsassociated with such lithiation-delithiation processes at higher voltage(>4.4 V) are not fully understood yet.

SUMMARY OF THE INVENTION

A lithium ion battery includes a positive electrode comprising carbonfibers, a binder composition comprising conductive carbon, and a lithiumrich composition. The lithium rich composition comprises at least oneselected from the group consisting of Li_(1+x)(My Mz^(II) Mw^(III))O₂where x+y+z=1, and xLi₂MnO₃(1−x)LiMO₂, where x=0.2-0.7, and where M,M^(II) and M^(III) are interchangeably manganese, nickel and cobalt, andLiM_(2−x)M_(x) ^(II)O₄, where M and M^(II) are manganese and nickel,respectively, with x=0.5. A negative electrode comprises carbon fibershaving bound thereto silicon nanoparticles, and a mesophase pitchderived carbon binder between the silicon nanoparticles and the carbonfibers. An electrolyte is interposed between the positive electrode andthe negative electrode.

The carbon fiber of the positive electrode can be carbon nanofiber, thebinder composition can comprises a polymer binder and conductive carbonparticles, and the carbon nanofiber, polymer binder and conductivecarbon particles can be coated onto a metal current collector.

The carbon fiber of the positive electrode can be a carbon fiber mat.The binder composition can comprise a conductive mesophase pitch derivedconductive carbon binder.

The silicon nanoparticles can be between 50-100 nm. The total volumetricenergy density of the battery can exceed 600 Wh/L. The total gravimetricenergy density of the battery can exceed 400 Wh/Kg. The battery can becycled to a voltage greater than 4.6 volts.

The carbon binder can be derived from thermal decomposition of mesophasepetroleum pitch. The loading of silicon nanoparticles on the fibers canbe between 25 and 50% by weight. The thickness of the cathode can bebetween 75-150 μm. The thickness of the silicon-carbon fiber anode canbe >100 microns and the silicon loading can be between 25-50%.

The anode can have an initial capacity that exceeds at least by 10% thecapacity of the cathode layer when normalized to thickness and area ofthe electrode. In one aspect either or both electrodes does not comprisea metal current collector. The electrodes in another aspect do notcomprise a polymeric binder.

The lithium rich composition can beLi_(1.2)Mn_(0.525)Ni_(0.175)Co_(0.1)O₂. The lithium rich composition canbe LiMn_(1.5)Ni_(0.5)O₄.

The cathode primary particle sizes can be below 100 nm. The carbonnanofibers can have an outer diameter of between 100 and 200 nm. Thecarbon fibers for the anode can have a diameter between 2-25 μm. Thecarbon nanofibers can have a hollow core of from ½ to ⅔ of the totalfiber diameter.

A method of making a positive electrode includes the step of providingthe following constituents: (i) carbon fibers; (ii) a lithium richcomposition; (iii) a binder composition with conductive carbon; (iv) anda solvent. A homogenous slurry of at least the binder composition andthe lithium rich composition is created. The slurry can be coated ontothe surface of a metal current collector such as aluminum or onto acarbon fiber mat. In one aspect the carbon fibers are carbon nanofibers,and the binder composition comprises a polymeric binder and conductivecarbon particles. In another aspect, the carbon fibers are provided as amat and the binder composition comprises a conductive mesophase pitchderived binder. The slurry is then thermally treated to carbonize thepitch and form a composite electrode. The cathode can comprise carbonfibers, mesophase pitch and the lithium rich composition.

The lithium rich composition can be between 85-90%, the mesophase pitchcan be between 5-7%, and the carbon nanofibers can be between 0.5-1% byweight of the electrode.

The solvent can be any suitable solvent, such as N-vinyl pyrolidonne.The thermal treatment can be between 500-700° C.

A method of making a negative electrode includes the step of providingthe following constituents: (i) carbon fibers; (ii) siliconnanoparticles; (iii) a mesophase petroleum pitch; and (iv) a solvent. Ahomogenous slurry of the constituents (i)-(iv) is created. The slurry iscoated onto the surface of a carbon fiber matrix. The slurry is thenthermally treated to carbonize the pitch and form a composite electrode.

The carbon fibers can be between 40-50%, the silicon nanoparticles canbe between 40-50%, and the mesophase pitch can be between 5-7% relativeto the total electrode weight. The solvent can be N-vinyl pyrolidonne.The thermal treatment can be between 500-700° C.

BRIEF DESCRIPTION OF THE DRAWINGS

There are shown in the drawings embodiments that are presently preferredit being understood that the invention is not limited to thearrangements and instrumentalities shown, wherein:

FIG. 1 is a plot of voltage (V) vs. Li/Li⁺ vs capacity (mAhg⁻¹) for ahalf cell.

FIG. 2 is a plot of capacity (mAh) vs. cycle number for a full cell.

FIG. 3 is a plot of cell voltage vs. capacity (mAh).

FIG. 4 is a plot of capacity (mAh) vs. cycle number.

FIGS. 5(a)-(c) are scanning electron microscopy images of asilicon-carbon fiber composite and varying magnifications.

FIG. 6 is a scanning electron microscopy image of a silicon-nanoparticleand carbon binder.

FIG. 7 is a plot of intensity (a.u.) vs. Raman shift (cm⁻¹).

FIG. 8 is a plot of capacity (mAhg⁻¹) vs. cycle number for Si—C anode.

FIG. 9 is a plot of capacity (mAhg⁻¹) vs. cycle number.

FIG. 10 is a plot of capacity (mAhg⁻¹) vs. cycle number, and coulombicefficiency vs. cycle number.

DETAILED DESCRIPTION OF THE INVENTION

A lithium ion battery according to the invention includes a positiveelectrode comprising carbon fibers, a binder composition comprisingconductive carbon, and a lithium rich composition. The lithium richcomposition comprises at least one selected from the group consisting ofLi y M_(1+x)(M_(y) M_(z) ^(II) M_(w) ^(III))O₂, where x+y+w+z=1.Alternatively these compositions can be given as xLi₂MnO₃(1−x)LiMO₂where M=Mn,Ni,Co and x=0.2-0.7 and LiM_(2−x)M_(x) ^(II)O₄, where M,M^(II) are manganese and nickel, respectively, with x=0.5. A negativeelectrode comprises carbon fibers having bound thereto siliconnanoparticles. A mesophase pitch derived carbon binder binds the siliconnanoparticles to the carbon fibers, and binds the carbon fiberstogether. An electrolyte is interposed between the positive electrodeand the negative electrode.

The carbon fiber of the positive electrode can be carbon nanofiber, thebinder composition can comprises a polymer binder and conductive carbonparticles, and the carbon nanofiber, polymer binder and conductivecarbon particles can be coated onto a metal current collector. The metalcan be any suitable metal, such as aluminum.

The carbon fiber of the positive electrode can be a carbon fiber mat.The binder composition can comprise a conductive mesophase pitch derivedconductive carbon binder.

The lithium rich composition in one aspect can beLi_(1.2)Mn_(0.525)Ni_(0.175)Co_(0.1)O₂. The other lithium based cathodecomposition in another aspect is the case of high voltage spinel,LiMn_(1.5)Ni_(0.5)O₄. The cathode primary particle sizes can be below100 nm but can have secondary or aggregated particle sizes ranging up toseveral microns.

The carbon nanofibers can have an outer diameter of between 100 and 200nm. The carbon nanofibers can have a hollow core of from ½ to ⅔ of thetotal fiber diameter.

The carbon fibers for the anode can have a diameter in the range of 5-25μm, or between 8-10 μm.

The silicon nanoparticles can be between 50-100 nm. The invention isalso extendable to other high capacity metal alloy anodes such as Sn andCu—Sn and Sn—Co nanoparticle compositions. The loading of the siliconnanoparticles on the fibers can be between 30 and 50% of the totalelectrode mass per unit area and allowing about 50-70% void spaces inthe electrode to accommodate the volume changes upon lithiation.

The thickness of the cathode can range anywhere between 75-150 μm. Thethickness of the silicon-carbon fiber anode can be >100 μm.

The battery of the invention can have a total volumetric energy densityexceeding 600 Wh/L. The total gravimetric energy density of the batterycan exceed 400 Wh/Kg. The battery can be cycled to a voltage greaterthan 4.6 volts.

The carbon binder can be derived from the thermal decomposition ofmesophase petroleum pitch. The thermal treatment is selected tocarbonize the pitch. In one aspect, the thermal treatment is between500-700° C.

The battery can be designed such that the anode electrode layer has aninitial capacity that exceeds at least by 10% greater than the cathodewhen normalized to similar thickness and area of the electrode. This isachieved by adjusting the wt % of silicon loading on the C-fiber.

The battery of the invention can be constructed without a metal currentcollector. Either or both of the electrodes can be constructed without ametal current collector. Also, the battery of the invention can beconstructed with electrodes does not comprise a polymeric binder.

Any suitable electrolyte for lithium ion batteries can be used. In oneembodiment the electrolyte can be a mixture of ethylene carbonate andethyl methyl carbonate (EMC/Dimethyl carbonate (DMC) (3:7 wt %) in LiPF₆(1M) and can include high voltage electrolyte additive such as 1% HFiP(Tris(hexafluoro isopropyl)phosphate. Other electrolyte salts that canbe used include LiClO₄, lithium bis(trifluoromethanesulfonyl)imide(LiTFSI), lithium bis(oxatlato)borate (LiBOB), dissolved in mixtures oforganic solvents, including linear and cyclic carbonates such apropylene carbonate, dimethyl carbonate, and others. Other electrolytecompositions or additives can include fluorinated compounds as highvoltage electrolyte compositions such as fluorinated ethylene carbonate(EC), fluorinated ether-EC, fluorinated linear carbonates, fluorinatedether, fluorinated linear sulfone, fluorinated sulfone, tetramethylsulfone (TMS), and sulfone-silane hybrid electrolytes.

A method of making a positive electrode according to the inventionincludes the steps of providing (i) carbon fibers; (ii) a lithium richcomposition; (iii) a binder composition with conductive carbon; (iv) asolvent; creating a homogenous slurry of at least the lithium richcomposition and the binder composition; coating the slurry onto asurface; and thermally treating the slurry to form a compositeelectrode. In one aspect the binder composition comprises a polymerbinder and conductive carbon particles, and the carbon fiber is carbonnanofiber. This slurry can be coated onto a metal current collector suchas aluminum. In another aspect, the binder composition comprisesmesophase pitch and the carbon fibers are provided as a mat, and aslurry of the pitch and lithium rich composition is coated onto the mat.

The lithium rich composition can be 85-90%, the binder compositioncomprises mesophase pitch 5-7%, and the carbon fibers are nanofibers0.5-1%, by total weight of the positive electrode.

The solvent can be any suitable solvent. The solvent can be an organicsolvent. The solvent can be N-vinyl pyrolidonne.

The invention can utilize commercially prepared loose carbon fiber matcarbon fiber mat for supporting the active material. These mats are madefrom graphitic as well as nongraphitic carbon. One of the fibers that isused in this invention is derived from PAN (polyacrylonitirile) basedprecursor.

The slurry can be poured onto the carbon fiber mat and then thermallytreated to carbonize the mesophase pitch binder. This will bind thecarbon fibers and lithium rich composition together, and will adherethese materials to the carbon fiber mat to form the positive electrode.Any suitable carbonization process can be utilized. In one embodiment,the carbonization step is a thermal treatment at between 500-700° C.

A method of making a negative electrode includes the steps of providing(i) carbon fibers; (ii) silicon nanoparticles; (iii) a mesophasepetroleum pitch; (iv) a suitable solvent; creating a homogenous slurryof the constituents (i)-(iv); coating the slurry onto the surface of acarbon fiber matrix; and thermally treating the slurry to carbonize thepitch and form a composite electrode. Carbonizing the mesophase pitchwill bind the silicon particles to the carbon fibers, and will adherethe anode materials to the anode carbon fiber mat.

EXAMPLES

Preparation of Silicon-Carbon Fiber Electrodes

Poly acrylonitrile based non graphitic carbon fibers are used as currentcollector for preparation of silicon anodes. The surface area of fibersis 0.7 m²/g, as measured by N₂ adsorption. As received, the carbonfibers were in the form of fiber mat. The fiber substrate was coated byspreading with slurry of P—pitch (50 weight %, from Cytec IndustriesInc., USA) and Silicon nano powder (Size ˜100 nm, 50 weight %, fromAldrich) in N-vinylpyrrolidone (NVP). The slurry of P—pitch and Siliconin NVP was prepared by thorough mixing of the material by high-energyball milling (model 8000M Mixer/Mill) and mixing (SPEX SamplePrep,Metuchen, N.J., USA). During processing development, the pitch-to-Sipowder ratio was kept constant (1 to 1). After coating, any excessslurry was carefully removed from the surface and the sheet was dried at90° C. Typical loading of Silicon powder to CF mat achieved with thistechnique is approximately 40-50% by weight and is limited by theviscosity of the slurry. The sheets were then pressed at 1 Ton/cm². Thepressed mats were then punched into desired coin cell size of area ˜1cm² followed by heating at 700° C.-100° C. under Ar atmosphere for 5 hto carbonize the petroleum pitch.

Preparation of Li-Rich NMC Electrodes

A typical process for high voltage cathode fabrication is given belowLi-rich NMC composite electrodes were prepared with anN-methylpyrrolidone (NMP) (Aldrich, 99.5% purity) slurry of Li-rich NMC,PVDF (Aldrich), C-black (CB) (Super P), carbon nano fiber in wt. % ratioof 85:7.5:6:1.5 coated on to Al foil using doctor blade. The coated Alfoil was the dried under vacuum at 90° C. for about 12 hrs followed bycalendaring at pressure of 1 Ton/cm². The electrodes comprised ˜10 mg ofactive Li-rich MNC per cm² on Al (Alfa Aesar, 99.99% purity) currentcollector.

FIG. 1 is a plot of voltage (V) vs. Li/Li⁺ vs capacity (mAhg⁻¹) for ahalf cell. Both 1^(st) cycle and C/10 curves are shown.

FIG. 2 is a plot of capacity (mAh) vs cycle number for a full cell. Thecell is a two electrode coin-type cell configuration with EC-DMC 1:2/1.2M LiPF₆ as the electrolyte. This plot shows increasing capacity athigher cycling voltage and upon prolonged cycling.

FIG. 3 is a plot of cell voltage vs. capacity (mAh) for the full cellLithium rich MNC and Si—C. This plot demonstrates a capacity between2-4.6 volts.

FIG. 4 is a plot of capacity (mAh) vs. cycle number for Li-rich NMC-Si—Cfiber full cell and demonstrates stable capacity both on charge anddischarge with cycling up to almost 50 cycles.

FIGS. 5(a)-(c) is a scanning electron microscopy images of asilicon-carbon fiber composite and varying magnifications. The figuresdemonstrate good porosity for electrolyte and ion transport.

FIG. 6 is a scanning electron microscopy image of a silicon-nanoparticleand carbon binder. This image demonstrates how the pitch-based carbonbinder coats the silicon nanoparticles to adhere them to the carbonfibers.

FIG. 7 is a plot of intensity (a.u.) vs Raman shift (cm⁻¹). The presenceof the D band (1350 cm-1) and G (1590 cm-1) band indicate the presenceof amorphous carbon and fiber and Si at 520 cm⁻¹.

FIG. 8 is a plot of capacity (mAhg⁻¹) vs. cycle number forsilicon-carbon fiber electrodes. The data shows high capacity for Si—Canode when cycled to 5 mV (measured in a half cell configuration with Limetal as counter-electrode), When cycled up to only 100 mV the capacityis around 400 mAh/g but very stable cycle life.

FIG. 9 is a plot of capacity (mAhg⁻¹) vs. cycle number at a number ofdifferent charge and discharge rate for the full Si—C—Li rich NMC cell.

FIG. 10 is a plot of capacity (mAhg⁻¹) vs. cycle number and coulombicefficiency vs. cycle number. The coulombic efficiency plot at the top ofthe chart indicates good lithium intake.

This invention can be embodied in other forms without departing from thespirit or essential attributes thereof, and accordingly reference shouldbe had to the following claims as indicating the scope of the invention.

We claim:
 1. A lithium ion battery, comprising: a positive electrodecomprising an integrated conductive non-graphitic carbon fiber matcurrent collector and a positive electrode composition, the positiveelectrode composition comprising a binder composition and a lithium richcomposition, the binder composition comprising a mixture of carbonizedpitch and conductive carbon nanofibers, and the lithium rich compositioncomprising at least one selected from the group consisting ofLi_(1+x)(M_(y) M_(z) ^(II) M_(w) ^(III))O₂ where x+y+z+w=1,xLi₂MnO₃(1−x)LiMO₂, where x=0.2-0.7, wherein M, M^(II) and M^(III) areinterchangeably manganese, nickel and cobalt, and LiM*_(2−x)M*_(x)^(II)O₄, wherein M* and M*^(II) are manganese and nickel, respectively,with x=0.5, the positive electrode comprising 85-90 wt. % lithium richcomposition, 5-7 wt. % carbonized pitch, and 0.5-1 wt. % carbonnanofibers by total weight of the positive electrode, the conductivenon-graphitic carbon fiber mat serving as a support for the positiveelectrode composition and as a current collector for the positiveelectrode, the thickness of the positive electrode being between 75-150μm; a negative electrode comprising an integrated conductivenon-graphitic carbon fiber mat current collector having bound theretosilicon nanoparticles, and a binder composition comprising a mixture ofcarbonized pitch surrounding the silicon nanoparticles and between thesilicon nanoparticles and the carbon fiber mat, the conductivenon-graphitic carbon fiber mat serving as a support for the siliconnanoparticles and as a current collector for the negative electrode, thethickness of the anode being greater than 100 μm; the carbon fibers ofthe conductive non-graphitic carbon fiber mats having a diameter between2-25 μm, and the carbon nanofibers having a diameter between 100-200 nm,and the void space in the electrodes being 50% to 70%; and anelectrolyte interposed between the positive electrode and the negativeelectrode, wherein the battery has a nominal voltage greater than 3.75volts, can be cycled to a voltage greater than 4.6 volts, and whereinthe total gravimetric energy density of the battery exceeds 400 Wh/Kg.2. The battery of claim 1, wherein the binder composition of thepositive electrode, the negative electrode, or both the positive and thenegative electrode, comprises a mesophase pitch derived conductivecarbon binder.
 3. The battery of claim 1, wherein the siliconnanoparticles are between 50-100 nm in diameter.
 4. The battery of claim1, the total volumetric energy density of the battery exceeds 600 Wh/L.5. The battery of claim 1, wherein the loading of silicon nanoparticleson the carbon fibers of the carbon fiber mat of the negative electrodeis between 30 and 50% of the total electrode mass per unit area.
 6. Thebattery of claim 1, wherein the silicon loading of the negativeelectrode is between 30-50% of the total electrode mass per unit area.7. The battery of claim 1, wherein the negative electrode has an initialcapacity that exceeds at least by 10% the capacity of the positiveelectrode when normalized to thickness and area of the electrode.
 8. Thebattery of claim 1, wherein the electrodes do not comprise a polymericbinder.
 9. The battery of claim 1, wherein one of the lithium richcomposition is Li_(1.2)Mn_(0.525)Ni_(0.175)Co_(0.1)O₂.
 10. The batteryof claim 1, wherein the lithium rich composition isLiMn_(1.5)Ni_(0.5)O₄.
 11. The battery of claim 1, wherein the lithiumrich composition is provided as particles with particle sizes below 100nm.
 12. The battery of claim 1, wherein the carbon nanofibers of thepositive electrode have an outer diameter of between 100 and 200 nm. 13.The battery of claim 1, wherein the carbon nanofibers of the positiveelectrode have a hollow core of from ½ to ⅔ of the total fiber diameter.